1. Field of the Invention
The present invention relates to performing distributed measurements in a borehole penetrating the earth. More particularly, the measurements are performed using an optical reflectometer.
2. Description of the Related Art
In exploration and production of hydrocarbons, it is often necessary to drill a borehole into the earth to gain access to the hydrocarbons. Equipment and structures, such as borehole casings for example, are generally disposed into a borehole as part of the exploration and production. Unfortunately, the environment presented deep into the borehole can place extreme demands upon the equipment and structures disposed therein. For example, the equipment and structures can be exposed to high temperatures and pressures that can effect their operation and longevity.
In order to monitor the health of the equipment and structures disposed downhole, a fiber-optic distributed sensing system (DSS) may be used. Sensing fiber (an optical fiber containing sensors or in itself functioning as a sensor or sensors may be attached to the equipment and structures at various locations usually at different depths in the borehole. The sensors can measure temperature, pressure, strain, and other parameters. By measuring strain for example, the system can determine if borehole casing is being deformed.
In one type of DSS optical frequency domain reflectometry or swept-wavelength interferometry can be used to interrogate a series of fiber Bragg gratings. Each fiber Bragg grating (FBG) in the series acts as a sensor. The optical fiber, in one example, is affixed to casing wrapped along a length of the casing. As each FBG is exposed to a changing condition, the optical characteristics of each FBG will change in relation to the changed condition. A sensor interrogator is used to measure the optical characteristics of each of the FBGs in order to ascertain the changing conditions.
FIG. 1 presents an example of a conventional Optical Frequency Domain Reflectometry (OFDR) system. In this example, the optical fiber includes a reference reflector and a series of FBGs. A swept-wavelength light source is coupled to the fiber. The wavelength of light from the light source is swept to interrogate each of the FBGs. The reference reflector forms an interferometric cavity, such as a Fabry-Perot cavity in this example, with each individual FBG.
As the wavelength of light from the light source is swept, an interferogram is created with a frequency for each interferometric cavity that is proportional to the length of the cavity for each FBG. Thus, spectral data from each FBG is modulated with a unique frequency, which ultimately permits individual inspection of the FBGs through conventional signal processing techniques. Converting the spectral data into the spatial frequency domain through a Fast Fourier Transform yields a view of the fiber that is the amplitude of the reflected light as a function of distance. In this manner, each FBG can be monitored and treated as an individual sensor.
In the conventional sensing fiber depicted in FIG. 1, the fiber is configured in such a way that the length of “blank fiber” (i.e., optical fiber with no FBGs) is approximately the same as the length of fiber with FBGs. This blank fiber is located between the reference reflector and the FBGs to ensure that autocorrelation terms, i.e., those reflections resulting from FBGs interfering with other FBGs in the fiber, are located in the lower band of the spatial frequency domain. Thus, undesirable autocorrelation terms are separated from desirable FBG profiles, thereby, removing the corrupting effects.
Unfortunately, with the conventional OFDR system, only “X” distance of the sensing fiber can provide for sensing as depicted in FIG. 1. Sensing lengths of the fiber cannot be made arbitrarily long because of constraints resulting from the necessity to digitize the data coming back from the sensing fiber. For example, as the effective optical distance between a FBG and a reference reflector increases, the frequency of the modulation from that FBG also increases. Thus, a practical limit on the sensing length is the speed at which the signals from the FBGs can be sampled. Additionally, it is typically true that the longer the sensing length, the more susceptible the FBG signals are to corruption from vibration of the sensing fiber.
Therefore, what are needed are techniques to increase the sensing length of an optical fiber for OFDR. Preferably, the sensing length is increased without incurring penalties due to higher sampling requirements or increased susceptibility to vibration.